Linking atmospheric mercury to methylmercury in fish

(09.10.2006)

Scientists have, for the first time, demonstrated a direct link between the amount of mercury being loaded into a lake by atmospheric deposition and the amount of toxic methylmercury that accumulates in fish swimming in the lake.

BackgroundA significant part of mercury in the atmosphere today is generated from human activity such as power plant emissions, and industrial and mining activities. Once deposited to the aquatic environment, bacteria, being present in the sediments of lakes and wetlands are responsible for the conversion of inorganic mercury into methylmercury, an organic form of the metal that accumulates in fish and is and is toxic to humans and fish-eating wildlife. But scientists have not been able to make a definitive link between the amount of anthropogenic mercury in air and the levels of methylmercury reaching the foodweb. A so called "mercury paradoxon" which means that fish swimming in waters with very different levels of mercury contamination can have similar methylmercury concentrations troubles the picture.

New research resultsNow, a study published in the recent issue of Environmental Science & Technology (Vol. 40/19, pp 5992–6000) suggests that reducing the amount of mercury that rains down on lakes also decreases methylmercury production. This is an important finding because many areas already have large historical accumulations of mercury in sediments and soils, and it has been difficult to distinguish between the impacts of historical contamination and new deposition.

In the study, led by Diane Orihel of Fisheries and Oceans Canada, researchers added isotope-enriched mercury to 11 lake enclosures called mesocosms (see Photo to the left) located at the Experimental Lakes Area in northern Ontario (Canada). The extra mercury increased levels of the heavy metal to 2–15× the typical local deposition rate. The boreal lakes of northern Ontario receive relatively low levels of mercury deposition—water bodies in the northeastern U.S. receive ~4× more, according to Orihel.

The researchers found that after 8 weeks, <1% of the isotope-enriched mercury was converted to methylmercury, while a much higher percentage was reemitted to the atmosphere. However, Orihel emphasizes that even a small percentage of methylmercury is harmful. “It may be a tiny amount, but that is all it takes to drive those fish advisories,” she says.

James Hurley, an aquatic chemist with the University of Wisconsin Sea Grant Institute, says the study shows that decreasing atmospheric loading should lower methylmercury production and, presumably, bioaccumulation in fish.

And Hammerschmidt adds that his own studies bolster this conclusion. In Alaskan lakes, he and colleagues measured loadings of inorganic mercury and the flux of methylmercury out of the sediments. The relationship is linear, he reported in August at the Eighth International Conference on Mercury as a Global Pollutant. Last year, Hammerschmidt found a similar relationship between methylmercury in mosquitoes and average deposition rates of mercury in U.S. lakes (Environ. Sci. Technol. 2005, 39, 3034–3039).

At the August meeting, Orihel reported the same linear relationship for small fish. Her team found that isotope-labeled mercury accounted for up to a third of the total concentration of mercury in the muscle tissues of young trout living in the lake mesocosms.

High methylmercury concentrations in North American freshwater fish have prompted health authorities in Canada and most U.S. states to warn against eating too much of the fish. Coal- and oil-fired power plants are the largest sources of mercury emissions in the U.S., according to the U.S. EPA.

But emissions controls are controversial, in part because of the complexity of the mercury cycle. In March 2005, EPA adopted a cap-and-trade rule, which aims to reduce mercury emissions 21% by 2010 and 69% by 2018. The new study suggests that limits could achieve their intended outcomes rather quickly.